In many aircraft, main propulsion engines not only provide propulsion for the aircraft, but may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical and/or pneumatic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main propulsion engines may not be capable of supplying the power needed for propulsion as well as the power to drive these other rotating components. Thus, many aircraft include an auxiliary power unit (APU) to supplement the main propulsion engines in providing electrical power to electrical loads and/or bleed air to pneumatic loads. An APU may also be used to start the propulsion engines.
An APU is typically a gas turbine engine that includes a combustion section, a power turbine section, and a compressor section. During operation of the APU, the compressor section draws in and compresses ambient air and supplies the air to the combustion section. Fuel is injected into the compressed air within the combustion section to produce the high-energy combusted air to the power turbine section. The power turbine section rotates to drive a generator for supplying electrical power, via a main shaft, and to drive its own compressor section and/or an external load compressor.
When needed, compressed air may be bled from the compressor in the APU via a bleed air port and a load control valve. The load control valve may be configured as either a modulating-type valve or an open/closed-type valve. When a modulating-type valve is used, the load control valve is used to, among other things, limit the pneumatic load on the APU. The load control valve is typically controlled via an electronic control unit that implements closed-loop feedback control based on APU exhaust gas temperature (EGT). When an open/closed-type valve is used, the APU typically relies on downstream devices to limit the pneumatic load. If the flow capacity of the downstream device has a larger capacity than the APU, there is risk of over temperature on the APU. The APU will close the load control valve in the event of an over temperature resulting in loss of bleed air to the aircraft.
One of the main pneumatic loads for an APU is the aircraft environmental control system (ECS). As is generally known, an aircraft ECS is either sized such that it will not exceed APU flow capacity (as indicated by its exhaust gas temperature) or has a means to self-regulate its flow capacity so as to not exceed APU flow capacity. Typically, an ECS system is configured to regulate its input flow utilizing an inlet flow control valve that is controlled independently of the APU. For example, the inlet flow control valve may be controlled based on measured airflow into the ECS. Such flow regulating methodologies typically require relatively large margins to account for airflow measurement tolerances, and on the APU's power capability to ensure that ECS flow demand does not exceed APU flow capacity. These relatively large margins limit the actual power utilization capability of the APU.
Hence, there is a need for a system and method for controlling bleed air inlet flow into an aircraft ECS system that does not rely on relatively large margins, so that ECS demand and APU capacity can be more closely matched. The present invention addresses at least this need.